Method of and an equipment for optical inspection of strip and sheet products

Abstract

A method of and an equipment for optical inspection of strip and sheet products for the detection of surface defects, which equipment comprises an image forming apparatus 1, 2 for producing images of successive parts of the surface of a product 3 and for converting the images to analog signals 4, an A/D converter 5 for converting the analog signal to a digital signal 6, and an image analyzing unit 7 for analyzing the digital signal to detect surface defects therefrom. To reduce the data processing capacity required of the image analyzing unit, the equipment further comprises a compression unit 8 arranged between the A/D converter and the image analyzing unit for compressing the digital signal before it is analyzed.

I claim: 
     
       1. A method of optical inspection of strip and sheet products for the detection of surface defects, which method comprises: producing images of successive parts of the surface of a product,   converting the images produced to an analog signal on the basis of the brightness of pixels,   converting the analog signal to a digital signal by providing each pixel with a digital value representing its brightness,   comprising the digital signal,   analyzing the compressed digital signal for the detection of surface defects on the product, and   giving information of the surface defects detected, wherein at the compression the brightness value of each pixel is compared with n/2 preceding and n/2 following pixel brightness values in a scanning direction, and replaced by one of the minimum and the maximum brightness value observed at the comparison, depending on whether the surface defects show dark or light from their surroundings, and every nth value is selected from the sequence of values thus obtained and used for the production of the compressed signal.   
     
     
       2. An equipment for optical inspection of strip and sheet products for the detection of surface defects, comprising: an image forming apparatus (1, 2) for the production of images of successive parts of the surface of a product (3) and for the conversion of the images to an analog signal (4), an A/D converter (5) for the conversion of the analog signal to a digital signal (6), a compression unit (8; 20) for the compression of the digital signal from th A/D converter, and an image analyzing unit (7) for analyzing the digital signal to detect surface defects therefrom, wherein the compression unit (8) comprises n delay (10) connected in series and n reference units (11) connected in series, and a register (12) arranged after the nth reference unit, the series connection of the delay units being arranged to receive the digital signal (6) from the A/D converter and the series connection of the reference units also being arranged to receive the digital signal from the A/D converter, each reference unit additionally receiving an output signal (13) of a corresponding delay unit in order, and the register being arranged to output every nth signal value. 
     
     
       3. A method of optical inspection of strip and sheet products for the detection of surface defects, which method comprises: producing images of successive parts of the surface of a product,   converting the images produced to an analog signal on the basis of the brightness of pixels,   converting the analog signal to a digital signal by providing each pixel with a digital value representing its brightness,   compressing the digital signal,   analyzing the compressed digital signal for the detection of surface defects on the product, and   giving information of the surface defects detected, wherein at the compression the pixels are divided into groups of n successive pixels, and one of the maximum and the minimum pixel brightness value is selected from each group, depending on whether the surface defects show dark or light from their surroundings, to be used for the production of the compressed signal, wherein from a group of n successive pixels one of the minimum and the maximum pixel brightness value is selected by comparing the first two pixel values with each other and selecting a smaller or greater one, by comparing the selected pixel value with the following pixel value and selecting a smaller or greater one, and so on, the result of the comparison with the last pixel of the group being the pixel of the minimum or the maximum value searched for.   
     
     
       4. An equipment for optical inspection of strip and sheet products for the detection of surface defects, comprising: an image forming apparatus for the production of images of successive parts of the surface of a product (3) and for the conversion of the images to an analog signal (4), an A/D converter (5) for the conversion of the analog signal to a digital signal (6), a compression unit (8,20) for the compression of the digital signal from the A/D converter, and an image analyzing unit (7) for analyzing the digital signal to detect surface defects therefrom, wherein the compression unit (20) comprises a series connection of a multiplexer (21), a delay element (22) and a register (12), the multiplexer being arranged to receive the digital signal (6) from the A/D converter and an output signal (25) of the delay element, and a comparator (23) arranged to receive the digital signal (6) from the A/D converter and the output signal of the delay element, and to control the multiplexer on the basis of the result of the comparison, and a control logic (24) arranged to control the register to output every nth signal value and the comparator to control, irrespective of the comparison performed by it, the multiplexer to deliver every n+1st signal value to its output.